1. Field of the Invention
The present invention relates to a circuit for detecting the end of life of a fluorescent lamp, and more particularly to a circuit for detecting the end of life of a fluorescent lamp in an early stage of the end of life so as to effectively protect a ballast, a socket, and relevant circuits of the fluorescent lamp.
2. Description of the Related Art
If a fluorescent lamp is used for a long time, Barium (Ba) or Strontium (Sr), the material of an auxiliary emitter for thermionic emission coated on an electrode (cathode) at each end of the fluorescent lamp, is absorbed into inner or outer walls of its tube and the amount of material coated on filaments of the lamp gradually decreases. When the amount of material coated on the filaments decreases below a certain level, the voltage and current of the fluorescent lamp become asymmetrical. If the coated material completely disappears, the lamp no longer turns on.
One can easily identify a lamp whose filament has been cut or fluorescent materials have completely disappeared since such a lamp no longer turns on. However, it is difficult to visually determine whether the lamp life is at the end before its fluorescent materials completely disappear. Thus, general users continue using a lamp even when the lamp is at the end of its life.
When a lamp is at the end of its life, heat is generated at an electrode with a reduced thermionic emission material due to an increased filament heat resistance and a constant electrode current. As the lamp approaches the end of its life, the temperature of the electrode gradually increases. If the electrode generates intense heat, the heat may melt a lamp socket connected to the electrode, thereby causing a dangerous situation. This is described below in more detail with reference to the drawings.
FIG. 1 illustrates the configuration of a so-called electronic ballast. A controller IC 10 can drive two transistor switches M1 and M2 alternately to generate a square wave at an output terminal VA. The square wave then passes through a filter including elements L, Cs, and Cp producing a sinusoidal wave to a fluorescent lamp. A discharge lamp, such as a fluorescent lamp, can have a negative resistance characteristic. This means that after turning on the lamp, the current continues to increase. To compensate this characteristic, discharge lamps require a circuit that prevents the continued increase of their lamp current and stabilizes the lamp current at a substantially constant level. This circuit is referred to as a ballast.
In order to provide and maintain an appropriate lamp current, some designs use an inductor with a very high inductance since the conventional ballast directly uses 60 Hz power. Therefore, the ballast needs a large size coil to provide adequate impedance. An electronic ballast has been developed to overcome this problem.
There are various types of electronic ballasts. One of the often-used ballasts is a half-bridge resonant inverter as shown in FIG. 1. The electronic ballast can reduce the size of the inductor by switching on and off a voltage VDC using the switches M1 and M2 to generate a high frequency AC voltage.
Before a lamp is turned on, the resistance between the electrodes of the lamp is very high. Once the lamp is turned on, the resistance between the electrodes of the lamp is reduced to hundreds of ohms. The characteristics of the resonant circuit including L, Cs, Cp, and the resistor between the electrodes vary as shown in FIG. 2 before and after the lamp is turned on. “fr” is the resonant frequency corresponding to the circuit elements L, Cs, and Cp in FIG. 1, and “fo” refers to the operating frequency of the switches M1 and M2 in FIG. 1. The curve labeled “Before ignition” refers to the resonant characteristic of the L, Cs, and Cp circuit elements before the lamp is turned on, whereas the “After ignition” refers to the same characteristic after the ignition. The frequencies “fignition” and “fpreheat” refer to the operating frequency during ignition and preheat, respectively. Both these steps occur before the lamp is turned on and thus correspond to the “Before ignition” curve. “frun” is the operating frequency after the lamp is turned on, and thus corresponds to the “After ignition” curve.
However, if a fluorescent lamp is used for a long time, the electron emitting material that is coated on the cathodes of the fluorescent lamp is gradually absorbed onto its tube, thereby reducing the amount of thermions emitted from the electrode into the tube. Here the term “thermions” refers to the electrons emitted from metals at high temperatures. This phenomenon is caused by adhesion of material such as Ba or Sr, coated on the electrodes of the lamp, to fluorescent materials on an inner surface of the tube. Accordingly, the electrodes of the fluorescent lamp that was used for a long time can blacken: this process is sometimes called blackening or darkening.
The blackening reduces the amount of current provided from the electrodes into the tube due to the reduced amount of thermions emitted from the electrodes. In addition, the amount of electron emitting materials on the electrodes of the fluorescent lamp may be reduced to different levels. As the lamp current, which flows from the electrodes of the fluorescent lamp into its tube, decreases the lamp voltage increases to a different level. Also, as the lamp current decreases, the brightness of the lamp also decreases. These signs are sometimes referred to as a fluorescent lamp end-of-life phenomena. Generally, in the early stage of the end of life of the fluorescent lamp, it is impossible to visually determine whether its brightness has been reduced.
If the life of a specific electrode (filament) of a fluorescent lamp is ended, then the amount of current flowing from the electrode to an opposite electrode is reduced. The lamp voltage varies inversely to the lamp current, as shown in FIG. 4.
If the end-of-life phenomenon continues, the extent of the asymmetry of the current and voltage of the fluorescent lamp increases. If the lamp current is reduced below the minimum current required to keep the lamp on, the lamp will not remain in an on state, but instead it will alternately turn on and off. Finally, if the electron emitting material is completely discharged from the electrodes of the fluorescent lamp, the lamp stays in the off state permanently.
In addition, if the amount of an electron emitting material on an electrode of the fluorescent lamp is reduced, the heat resistance of the electrode is increased. If an upper electrode in FIG. 1 is darkened, a current in direction Ia is decreased while current of direction Ib is increased. High heat is generated in the electrode of the fluorescent lamp due to the increased heat resistance of the electrode and the increased current of direction Ib. This heat can melt a plastic socket connected to the electrode of the lamp.
FIG. 3 illustrates a conventional method to determine whether or not a lamp is near the end of its life. In this conventional method, a voltage, obtained through division of a voltage between the electrodes of a lamp, is detected and, if the detected voltage is higher than a predetermined reference level, it is determined that the lamp is at the end of its life and the ballast is deactivated.
Although this method is advantageous in that it can easily detect the end of life of the lamp, it has a problem in that a protection circuit may operate abnormally depending on how passive elements are selected. In addition, if the lower electrode rather than the upper electrode is darkened in FIG. 3, a voltage increase caused by darkening cannot be detected with the voltage detected through the voltage division alone, thereby failing to correctly detect the end of life of the lamp.